Self-healing material and preparation process thereof

ABSTRACT

The present application provides a self-healing material which comprises silica sol as self-healing agent encapsulated by a polymeric shell. The self-healing material may be further embedded in a concrete mixture to heal micro-cracks in concrete. A method for preparing the self-healing material is also provided.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. patent applicationSer. No. 14/723,477 filed on May 28, 2015, which claims the benefit ofU.S. Provisional Patent Application No. 61/997,335 filed on May 29,2014, the entire content of which is hereby incorporated by reference.

FIELD OF TECHNOLOGY

The present application involves a self-healing material as well as aprocess for its preparation.

BACKGROUND

Concrete is a low cost and the most industrially used material. This isprimarily due to its high compressive strength and durability whichuntil now is considered to be an indispensable building material.However, due to long term influence of surrounding environment,micro-cracking and damage of the concrete material are inevitable.Micro-cracks in particular are not easily detected due to limitedtechnologies. Therefore, it is difficult to repair such undetectablecracks. When cracks are not restored timely and effectively, it leads tomacro-crack formation that triggers brittle fracture and shortensconcrete life, consequently threatens structure safety as well.Moreover, inspection and maintenance are difficult and expensive whereinthe labor cost goes up to 50% of the repair cost. Thus, timely repair ofmicro-cracks in concrete is of major concern.

One of the most common methods of addressing this problem in concrete isby use of self-healing agents in microcapsules embedded within theconcrete mix. Recently, examples of self-healing of concretes usingmicroencapsulation techniques have been reported. For example,microencapsulated sodium silicate has been used as a healing agent.However, the microencapsulated sodium silicate has low silica (SiO₂)content and is viscous. This results in less penetration intomicro-cracks and thus inefficient healing of concrete. Other existingself-healing concretes using hollow fibers and hollow glass tubes arenot feasible in commercial market. The use of concrete vibrator duringconstruction process leads to the destruction of hollow fiberarrangement and design and the premature loss of the self-healing agent.The use of bacteria and calcium lactate inside clay particle asself-healing agent yields a very expensive aggregate (e.g. Euro160/cubic meter) with an initial compressive strength 25% lower thannormal concrete. In general, microcapsules produced to date are notcheap enough for mass production. Synthetic conditions and parametersare not environmental-friendly and are difficult to scale-up. There isalso a problem of slow response between the microcapsules and thehealing mechanism during micro-cracking.

There is a need to produce a self-healing material which is efficient inhealing micro-cracks and suitable for mass production.

SUMMARY

In one aspect, the present application provides a self-healing materialcomprising a self-healing agent, and a polymeric shell encapsulating theself-healing agent, in which the self-healing agent comprises colloidalsilica.

In one embodiment, the weight percentage of silicon dioxide in thecolloidal silica may range from about 40% to about 50%.

In one embodiment, the polymeric shell may comprise at least one fromthe group consisting of polyurethane, polyurea, poly(urea-urethane),polystyrene, and urea-formaldehyde polymer.

In one embodiment, the self-healing material may comprise a microcapsulehaving a size of about 100 microns to about 200 microns.

In one embodiment, the polymeric shell may comprise a thickness of about30 microns to about 50 microns.

In one embodiment, the self-healing material may comprise a plurality ofmicrocapsules embedded in a concrete mixture. The microcapsule comprisesa self-healing agent encapsulated by a polymeric shell, and theself-healing agent comprises colloidal silica.

In one embodiment, the concrete mixture may comprise cement, sand,crushed rock and water.

In one embodiment, the concrete mixture may comprise cement, sand,crushed rock, water in a weight ratio of about 1:1-3:1-3:0.2-0.8.

In still another aspect, the present application provides a process forpreparing a self-healing material comprising a step of emulsifying aself-healing agent by at least two surfactants, in which theself-healing agent comprises colloidal silica, and a step ofencapsulating the emulsified colloidal silica to form a plurality ofself-healing microcapsules comprising a core of colloidal silica and apolymeric shell.

In one embodiment of the process, the weight percentage of silicondioxide in the colloidal silica ranges from about 40% to about 50%.

In one embodiment of the process, the step of emulsification comprisesemulsifying the colloidal silica in an organic phase may contain atleast two surfactants which may have a combining Hydrophile-LyophileBalance (HLB) of 3 to 5.

In one embodiment of the process, the surfactants may be selected fromthe group consisting of poly(ethylene glycol)-400 dioleate,poly(ethylene glycol)-8 dioleate, sorbitan laurate, poly(ethyleneglycol)-40 sorbitan peroleate, lecithin glycol distearate, sorbitantrioleate and propylene glycol isostearate.

In one embodiment of the process, the surfactants may comprisepoly(ethylene glycol)-400 dioleate and sorbitan trioleate.

In one embodiment of the process, the polymeric shell may comprise atleast one selected from the group consisting of polyurethane, polyurea,poly(urea-urethane), polystyrene, and urea-formaldehyde polymer.

In one embodiment of the process, the microcapsule may comprise a sizeof about 100 microns to about 200 microns, and the polymeric shell mayhave a thickness of about 30 microns to about 50 microns.

In one embodiment of the process, the emulsifying step may comprise astirring step at a speed of about 400-600 rpm.

In one embodiment of the process, the encapsulating step may comprise astirring step at a speed of about 700-950 rpm.

In one embodiment of the process, the process may further compriseadding surfactants.

In one embodiment of the process, the process may further compriseembedding a plurality of the self-healing microcapsules in a concretemixture.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments of the present application are described with referenceto the attached figures, wherein:

FIG. 1 shows a diagram representation of preparation of microcapsulesaccording to some embodiments of the present application.

FIG. 2 is a general procedure of preparing a self-healing concrete.

FIGS. 3(a) and 3(b) are SEM images of microcapsules from example 1containing self-healing agent in fresh (undiluted) and diluted formsrespectively.

FIG. 3(c) is a SEM image of a single microcapsule from example 1emphasizing its core and shell thickness.

FIG. 3(d) is a SEM image showing the silica particles inside themicrocapsule from example 1. The inset shows a cross-sectional view ofthe microcapsule.

FIG. 4(a) is an optical image of microcapsules from example 1.

FIG. 4(b) is an optical image of microcapsules from example 1 modifiedwith chitosan. The inset shows a single microcapsule.

FIG. 5 is FTIR spectrum of the microcapsule from example 1 showing itscomponents (e.g. silica and poly(urea-urethane)).

FIG. 6 is a nanoindentation analysis showing ultimate strength ofmicrocapsules from example 1.

FIG. 7 is a stress-strain diagram for poly(urea-urethane) film used aspolymeric shell in silica based microcapsules. Insets are figures thatderive a) elasticity modulus, b) yield strength, c) ultimate strength.

FIG. 8 is a graph illustrating the change of flexural strength ofself-healing concrete after cracking and healing.

FIG. 9 shows the initial compressive stress of self-healing concretewith three replicates and their average value.

FIG. 10 is a graph illustrating the change of compressive strength ofself-healing concrete after cracking and healing.

FIG. 11 is a graph showing the water absorptivity of the self-healingconcrete and control.

FIG. 12 is a graph showing the water absorptivity of the self-healingconcrete and control after micro-cracking.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details aredescribed in order to provide a thorough understanding of theapplication. However the present application may be practiced withoutthese specific details. In other instances, well known methods,procedures, components have not been described in detail so as not toobscure the present application.

Further, exemplary sizes, values and ranges may be given, but it shouldbe understood that the present application is not limited to thesespecific examples.

The present application provides self-healing materials, such as in aform of self-healing microcapsule or self-healing concrete, forrecovering strength during micro-cracking. Recovery of strength isgained when self-healing materials release the healing agent forming gelto fill the micro-cracks.

The present application also provides a cheaper, straight forward andindustrialized process for preparing a self-healing material.

The self-healing material of the present application may employ silicasol as the healing agent encapsulated by a protective polymeric shell toform microcapsules. The microcapsules may include at least silica sol, asurfactant, and a polymeric shell. Other additives such as emulsifier,filler, adhesive and thickener may also be present in the microcapsules.

Silica sol (or colloidal silica) is a colloidal system in which solidsilica particles are dispersed in a liquid solvent such as water. Silicasol may include different composition (%) of silicon dioxide.Commercially available silica sol is distributed by Sigma Aldrich with30 wt %, 34 wt %, 40 wt %, 45 wt %, and 50 wt % silicon dioxide contentsuspended in water. In some embodiments of the present application, thesilica sol may contain about 40-50 wt % silicon dioxide. It has beenfound that the percentage of silicon dioxide in the microcapsule mayaffect the quantity of the encapsulated material and thus the healingcapability of the microcapsules. Silica sol with less than 40 wt %silicon dioxide may result in a microcapsule not having enough silicondioxide particles. However, silicon dioxide content exceeding 50 wt %may be of excess and thus not economical for industrial production.

A common method for preparing silica sol from natural ore is through ionexchange process from liquid sodium silicate. Silica ore is mixed withan alkali solution (e.g. sodium hydroxide) and subsequently dissolved byheating and under pressure to produce liquid sodium silicate (commonlyknown as water glass). It then passes through an alkali solution to formsilica seeds for subsequent growth into particles. This is followed byconcentrating the product to 30% for commercial production.

Another method for preparing silica sol may be through hydrolysis andcondensation of tetraethoxysilane (TEOS), also known as Stober method.TEOS derived from natural silicon ore undergoes some modifications andforms colloidal silica by hydrolysis with acid and subsequentcondensation.

Alternatively, silica sol may be prepared from milling of fumed silicaor silica gel by zirconia beads in sand mill or nanofluidizer to reduceparticle size, followed by ultrasonic dispersion to form silica sol.

Silica sol may also be prepared by direct oxidation of metallurgicalgrade silicon (MG-silicon) without using TEOS. This may be carried outby treating MG-silicon with water in the presence of alkali catalystproducing colloidal silica with hydrogen and heat.

The shell of microcapsules of the present application may include one ormore polymers. Suitable polymers for use as the shell of microcapsulesmay include but not limited to polyurethane, polyurea,poly(urea-urethane), polystyrene, and urea-formaldehyde polymer.Precursors of polymer used for production of the shell of microcapsulesmay include a diol, a polyol, a polyether polyol, a polyester polyol, adiamine, a polyamine, a diisocyanate, a monomer containing both hydroxyland isocyanate functional groups, a monomer containing both amino andisocyanate functional groups, a monomer containing both hydroxyl andamino functional groups, styrene, divinylbenzene, urea and/orformaldehyde.

In some embodiments of the present application, the polymer of the shellof microcapsule may include polyurethane, polyurea and/orpoly(urea-urethane). In some other embodiments of the presentapplication, the polymer of microcapsule may includepoly(urea-urethane).

In some embodiments, the precursors of the polymer used for productionof the shell of microcapsule may include 4,4′-methylenebis(phenylisocyanate) (MDI), toluene diisocyanate (TDI), 1,6-hexamethylenediisocyanate (HDI), isophorone diisocyanate (IPDI), 4,4′-diisocyanatodicyclohexylmethane (HMDI), poly(ethylene glycol) dioleate (POEDO),PEG-8 dioleate, sorbitan laurate, PEG-40 sorbitan peroleate, lecithin,polyoxyethylene, sorbitan trioleate, glycol distearate, propylene glycolisostearate, ethylenediamine (EDA), methylene-bis-ortho-chloroaniline(MOCA), 2,4-diamino-3,5-dimethylsuphylchlorobenzene (DDSCB),3,5-diethyltoluenediamine, 1,6-hexane diamine (HMDA), 1,4-phenylenediamine (PDA), poly[N-(2,2-dimethoxy-1-hydroxy)] polyamines, mono-anddi-[N-(2,2-dimethoxy)-1-hydroxy)] urea, mono-, di-, tri-, and/ortetra-[N-(2,2-dimethoxy)-1-hydroxy)] melamine,di-[N-(2,2-dimethoxy)-1-hydroxy)] benzoguanidine,poly[N-(2-hydroxyacetaldehyde)] polyamines, mono-anddi-[N-(2-hydroxyacetaldehyde)] urea, mono-, di-, tri-, and/ortetra-[N-(2-hydroxyacetaldehyde)] melamine, poly[N-(2-hydroxyaceticacid)] polyamines, mono-and di-[N-(2-hydroxyacetic acid)] urea, mono-,di-, tri-, and/or tetra-[N-(2-hydroxyacetic acid)] melamine,di-[N-(2-hydroxyacetic acid)] benzoguanidine, poly[N-(ethane-1,2-diol)]polyamines, mono-and di-[N-(ethane-1,2-diol)] urea, mono-, di-, tri-,and/or tetra-[N-(ethane-1,2-diol)] melamine and/ordi-[N-(ethane-1,2-diol)] benzoguanidine.

In some embodiments, the precursors of the polymer used for productionof microcapsule may include 4,4′-methylenebis(phenyl isocyanate) (MDI),poly(ethylene glycol) dioleate (POEDO), sorbitan trioleate,poly[N-(ethane-1,2-diol)] polyamines, mono-and di-[N-(ethane-1,2-diol)]urea, mono-, di-, tri-, and/or tetra-[N-(ethane-1,2-diol)] melamineand/or di-[N-(ethane-1,2-diol)] benzoguanidine.

The microcapsules of the present application may be prepared by anycommon microencapsulation methods. Generally, microencapsulation methodsmay be divided into chemical methods and physical/mechanical methods.Chemical methods may include different polymerization techniques such assuspension polymerization, emulsion polymerization, dispersion andinterfacial polymerization. Physical/mechanical methods may includesuspension crosslinking, solvent evaporation/extraction,coacervation/phase separation, spray drying, fluidized bed coating, meltsolidification, precipitation, co-extrusion, layer-by-layer deposition,supercritical fluid expansion and spinning disk. In some embodiments ofthe present application, the microcapsules may be prepared byinterfacial polymerization.

According to some embodiments of the present application, themicrocapsules may be prepared in a one-pot-two-step synthesis as shownin FIG. 1. The self-healing agent, silica sol, may be emulsified in thepresence of surfactants and solvent under low speed of stirring. Aftercertain time of stirring, the mixture may be charged with a polymerprecursor and catalyst under high speed of stirring for catalyticin-situ interfacial polymerization. The resulting products may beuniform spherical microcapsules with a core-shell structure, with silicasol as the core and polymeric shell.

Suitable catalyst for catalytic polymerization of the microcapsule mayinclude bismuth carboxylates, zinc carboxylates, alkyl tin carboxylates,oxides and mercaptides oxides. In some more embodiments, the catalystmay be dibutyltin dilaurate (DBTDL) or dibutyltin dioctanoate.

Suitable solvent may include toluene, n-propyl propionate, methyln-propyl ketone, isobutyl isobutyrate, isobutyl alcohol, isobutylacetate and alipathic hydrocarbons.

Suitable surfactant for emulsification may include a diol, a polyol, apolyether polyol, a polyester polyol, a diamine, and/or a polyamine. Insome embodiments, the surfactant may include poly(ethylene glycol)dioleate (POEDO), PEG-8 dioleate, sorbitan laurate, PEG-40 sorbitanperoleate, lecithin, polyoxyethylene, sorbitan trioleate, glycoldistearate, and/or propylene glycol isostearate. In some moreembodiments, the surfactant may include poly(ethylene glycol) dioleate(POEDO) and/or sorbitan trioleate.

According to some embodiments of the present application, thesurfactants may be used to emulsify silica sol and serve as precursorsof the polymer used for production of microcapsules. In someembodiments, the surfactant for emulsification of silica sol may be amixture of surfactants. In some particular embodiments, the surfactantfor emulsification of silica sol may include a mixture of surfactants,each carrying functionality different from the other. In someembodiments, the surfactant may include a polyester polyol and apolyamine.

Selection and amounts of surfactants used in the emulsification ofsilica sol may be based on Hydrophile-Lyophile Balance (HLB) values ofthe surfactants. HLB value of a surfactant relates to its solubility.Low HLB value means oil-soluble while high HLB tends to bewater-soluble. Stability of the emulsion of silica sol can be determinedfrom the appearance of the emulsion and by observing phase separation bybare eyes and optical microscopy. It has been found that a mixture ofsurfactant having a (HLB) value ranging from about 3-5 may giveexcellent emulsion stability. In some embodiments, the HLB value of themixture of surfactants may be about 5.

Different weight ratios of surfactants may be used to come up withdifferent ranges in HLB values. The HLB value of a mixture ofsurfactants can be calculated using the following equation:

${HLB} = \frac{\begin{matrix}{{\left( {{{wt}.\mspace{11mu} {ratio}}\mspace{14mu} {of}{\; \;}S\; 1} \right)\left( {{HLB}\mspace{14mu} {of}\mspace{14mu} {pure}\mspace{14mu} S\; 1} \right)} +} \\{\left( {{{wt}.\mspace{11mu} {ratio}}\mspace{14mu} {of}\mspace{14mu} S\; 2} \right)\left( {{HLB}\mspace{14mu} {of}\mspace{14mu} {pure}\mspace{14mu} S\; 2} \right)}\end{matrix}}{100}$

wherein S1 and S2 represent a first surfactant and a second surfactantrespectively.

One of the parameters for preparing microcapsules is its size ordiameter. In the application of microcapsules in concrete, its diametermay affect its mechanical strength. Too small microcapsules are hard tobreak while too big microcapsules are easy to break. Thus, getting theright microcapsule size may be of significance to its application in aself-healing concrete. At an optimum size, self-healing microcapsulesare hard enough not to be broken easily by its surrounding environment,yet easy enough to rupture when micro-cracking occurs. A size range ofabout 100-200 microns may be used in this application. Three parameterswere investigated to optimize the silica-based self-healingmicrocapsules for applications in concrete, which are the stirring speedduring emulsification (emulsification speed), the stirring speed duringpolymerization (polymerization speed), and the monomer ratio.

It has been found that at higher emulsification speed (e.g. about1,000-12,000 rpm), silica sols may be in very fine white droplets andthe emulsion later separates into phases. This may be due to theincreased shear stress that silica sol droplets undergo duringemulsification. Thus, the emulsion system is not stable andprecipitation occurs. However, at lower emulsification speed (e.g. about50-500 rpm), optical images of the emulsion may show dispersed silicasol droplets. This proves that lower emulsification speed may result inmore stable and dispersed silica sol droplets. In some embodiments ofthis application, the emulsification speed may be in the range of about400-600 rpm. In some embodiments, the emulsification speed may be about500 rpm.

The effect of the polymerization speed on the size of microcapsules wasalso studied. Increasing polymerization speed may decrease the averagesize of microcapsules. In some embodiments of the present application, amicrocapsule particle size of about 820 microns may be obtained ataround 300 rpm while the size reduces abruptly when the polymerizationspeed was increased to 500 rpm. In some embodiments, a gradual decreaseof size of microcapsules from about 230 to about 94 microns may beobserved when the polymerization speed increased from about 500 rpm upto about 1,000 rpm. The microcapsule size of about 100-200 micron may beattainable at about 700-950 rpm. In some embodiments, the polymerizationspeed may be about 900 rpm. This also suggests that polymerization speedcontrols the equilibrium between shear forces and interfacial tension ofthe silica sol droplets and the local velocity gradient the dropletsexperience. This means that at low polymerization speed, interfacialtension is higher than shear force and thus produces largermicrocapsules. However, large microcapsules are broken into smaller oneswhen strong shear forces are higher under high polymerization speed.

In the polymerization process, aside from the effect of thepolymerization speed on the microcapsule size, its effects on smoothnessof the shell layer or coating size were also investigated.

In some embodiments of this application, when polymerization speed wasincreased from about 300 to 1,100 rpm, microcapsule evolved from a veryporous surface to a smooth surface. Big pores were observed at about 300rpm and reduced to smaller pores when speed was increased to about 500rpm. When speed was reached to about 700 rpm, particles may graduallybecome smoother up to a speed of about 1,100 rpm. This observation isattributed to the fact that higher stirring speed reduces theconglomeration of poly(urea-urethane) shell formation or the polymerdeposition on the droplet surface.

Results revealed that polymerization speed has a high influence on thestructure of microcapsules and that it can be controlled. It is idealthat microcapsules produced do not possess pores to prevent leaking ofthe core material, silica sol. In this regard, the polymerization speedmay be in the range of about 700-1,100 rpm. Another reason for thisphenomenon is due to the ability of the microcapsules to form longerchains of polymeric shell coating by means of reaction between thepolymer precursor (e.g. MDI) and the surfactant.

At constant emulsification and polymerization conditions, the amount ofpolymer precursor added during shell formation was also investigated. Ahigh core/shell ratio means a high amount of core material may bepresent in the microcapsule with a thin shell layer.

In some embodiments of this application, results showed that thecore/shell ratio decreases as the amount of polymer precursor increases.This means that the increasing addition of polymer precursor results toa thicker shell layer. Yields of microcapsules produced were alsocompared vis-a-vis its core/shell ratios. There was an increase in yieldwith an increase in polymer precursor used. From some embodiments, about10-20 wt %., and in particular, about 15 wt. %, of polymer precursor maygive the highest yield value with a minimum core/shell ratio. This meansthat at this condition, shell is at thickest possible. On the otherhand, encapsulation efficiency has no significant change on all polymerprecursor conditions.

The self-healing concrete of the present application may be prepared bythoroughly mixing concrete components (e.g. cement, sand, rock andwater) and healing agent components (e.g. plasticizer and microcapsules)in a cement mixer. General procedure of preparing the self-healingconcrete is illustrated in FIG. 2.

In some embodiments of this application, the concrete component maycomprise cement, sand, crushed rock and water in a weight ratio of1:1-3:1-3:0.2-0.8. The exemplary weight ratio of the concrete componentsin an embodiment is listed in Table 1.

TABLE 1 Component ratio (w/w) for preparing the concrete components.Cement Sand Rock Water 1 1.5 2 0.35

In some embodiments of this application, the healing agent componentsmay comprise plasticizers and microcapsules in a volume ratio of 0.5:1.The plasticizer may be present in the amount of about 1.5% wt. of thetotal weight of the self-healing concrete. The microcapsule may bepresent in the amount of about 5% of the total volume of theself-healing concrete.

Suitable plasticizer may include all commercially available plasticizersand superplasticizers. Examples of plasticizers include SP8 (also knownas MasterGlemium SKY 8588 or Glenium SP8S). Other suitable plasticizersthat can be applied for concrete are sulfonated melamine-formaldehydecondensates, sulfonated naphthalene-formaldehyde condensates, modifiedlignosulfonates and polycarboxylate derivatives. SP8 is asuperplasticizer for concrete that gives exceptionally high waterreduction and significantly reduces slump loss. The unique compositionof SP8 may stabilize the cement particles and disperse effectively. Thismay result to a flowable concrete and homogenous concrete.

Surfactants may be added in any suitable amount during preparation ofthe self-healing concrete. Suitable surfactants include all commerciallyavailable surfactants for preparation of concrete mixture. Examples ofsurfactants for concrete mixture include chitosan polyethyleneimine,polyethylene glycol bis (2-aminoethyl), poly(diallylammonium chloride),poly(acrylamide/2-methacyloxyehtytrimethylammonium bromide),poly(1-lysine hydrobromide), poly(N-vinylpyrrolidone), poly(vinylamine)hydrochloride, poly(2-vinylpridine), poly(2-vinylpyridine N-oxide),poly(N-vinylpyrrolidone/2-dimethylaminoethyl methacrylate),poly(-aminostyrene), poly(aniline), and poly(N-methylvinylamine),poly(allylamine hydrochloride).

Hereinafter the present application will be further illustrated by thefollowing non-limitative examples.

EXAMPLE 1

For preparation of microcapsules in a 100 mL beaker, 30 mL of aqueouscolloidal silica was emulsified in an organic phase containing toluene(64 mL) and 5 wt. % of binary mixture of POEDO and Span®85 (sorbitanetrioleate) at stirring speed of 500 rpm. After 10 minutes ofemulsification, polymerization was carried out by addition of a solutioncontaining a fix amount of monomer (15 wt. % of MDI) in toluene and twodrops of catalyst, DBTL, at 63° C. for 160 minutes at stirring speed of900 rpm to form polymeric crosslinked shell, encapsulating the silicacore material. The microcapsules produced were further washed twice withtoluene to remove the remaining MDI. The reagents used in emulsificationand polymerization are summarized in tables 2 and 3.

TABLE 2 Reagents used in emulsification of silica sol. 1. EmulsificationReagents Poly(ethylene glycol) 400 dioleate 2.95 g (POEDO) Span ®85(sorbitane trioleate) 3.05 g Toluene 64 mL Silica sol (50% w/w) 30 mLParameters Stirring Speed 500 rpm Time 10 min. Temperature r.t.

TABLE 3 Reagents used in polymerization of silica sol. 2. PolymerizationReagents 4,4′-Methylenebis (phenyl 3.2 g isocyanate) (MDI) (15% wt.)Toluene 10 mL Dibutyltin dilaurate (DBTL) 2 drops Parameters StirringSpeed 900 rpm Time 160 min. Temperature 63° C.

To investigate the morphologies and properties of the microcapsules,characterization of the microcapsules was performed through SEM, FTIR,optical microscopy and nanoindentation analyses.

Scanning Electron Microscope (SEM) samples were examined in JEOL-JSM6390 and 6300 models of Field Emission Scanning Electron microscope atan accelerating voltage of 15-20 kV. Samples were spread onto a coppersubstrate followed by air drying. A thin layer of gold film wassputtered on the dried sample under vacuum. FIGS. 3(a) and 3(b) are theSEM images of the self-healing microcapsules in fresh (undiluted) anddiluted forms respectively. From the images, fresh microcapsules are inuniform size (average size of 87 μm) and spherical shape, and highlymonodispersed. A single microcapsule is displayed in FIG. 3(c) toemphasize the components of the microcapsules. Here, the core and shellare evident based on the color contrast. A further examination wasperformed in a single microcapsule (FIG. 3(d)) wherein it shows thesilica particles inside the microcapsule. An inset SEM image shows thecross-sectional view of the microcapsule exposing the silica corecontent with the shell layer enveloping the core material.

Optical microscopy by dynamic mechanical analyzer (DMA 7, Perkin-Elmer)was used to characterize the mechanical properties of the polymericshell component of the silica-based microcapsules. The testing wascarried out at room temperature with a linear force of 8,000 mN on apoly(urea-urethane) film using a probe tip. FIG. 4(a) shows howmicrocapsules are seen in an optical microscope. It is evident that thecore component is in liquid state surrounded by a polymeric solid shell.FIG. 4(b) shows another optical microscope image of the self-healingmicrocapsule after being modified with chitosan which resulted to becompatible with water medium. In this image, there is no concreteinvolved but only the microcapsules with chitosan. The chitosan (5% wt.)to microcapsule volume ratio used is 20:2.

Fourier Transform Infrared (FTIR) Spectroscopy measurements wasperformed in an FTIR spectrophotometer (Bio-Rod FTS6000). Sample wasprepared by drying and mixing with KBr salt and pressed into disk,measured with a spectral range of 400-4000 cm⁻¹. From the FTIRspectroscopy of the microcapsules in FIG. 5, the appearance of the peakat 1705-1730 cm ⁻¹ represent carbonyl of the free and bonded urethanegroups while peaks 1660-1690 cm⁻¹ represent carbonyl for urea groups.Furthermore, N-H stretching at 3300-3450 cm⁻¹ and C—H asymmetrical at2920 cm⁻¹ also provide signatures for polyurethane groups. Silicondioxide groups are represented by triple peaks; bending at 472 cm⁻¹,stretching at 803 cm⁻¹ and vibration at 1100 cm⁻¹. Thus, themicrocapsule contains both the silica core and the polymeric shell ofpoly(urea-urethane).

A nanoindentation analysis of the prepared microcapsules was carriedout. The force-displacement relationship in three stages is shown inFIG. 6. First is the loading stage wherein force increases with depth ofthe microcapsule until it reaches its maximum load (140 mN) where forcebecame constant at certain holding time (second stage). The third stageis the sudden fall of the load at a certain depth of the microcapsule,indicating the rupture of the microcapsule and thus ultimate strength ofthe microcapsule.

The mechanical properties of the polymeric shell material (i.e.poly(urea-urethane)) was further analyzed by a dynamic mechanicalanalyzer (DMA 7, Perkin Elmer) wherein tensile stress was applied on apoly(urea-urethane) film sample and thus derived a stress-strain diagram(FIG. 7). FIG. 7 demonstrates both elastic and plastic properties, yieldstrength and ultimate strength of the poly(urea-urethane) film which isused as a polymeric shell layer of the self-healing microcapsules. Theelastic property of the film is depicted on the linear curve (FIG. 7inset a). This curve has a slope of 1.3 kPa, which corresponds to theelasticity modulus value. It shows that poly(urea-urethane) film, at acertain load or stress range, is proportional to the elongation orstrain it experiences. However, at a specific point or load, stressbecomes constant while strain increases (FIG. 7 inset b). This new curvederives the yield strength of 29 kPa and represents the plastic propertyof the poly(urea-urethane) film. Finally, FIG. 7 inset c shows a linearcurve from the peak of the original plot representing the ultimatestrength curve. Ultimate strength is derived from the peak point of thestress-strain curve. It has a value of 98 kPa.

EXAMPLE 2

12 pieces of beams (30 cm×10 cm×4 cm) for flexural strength wereprepared using the component ratio in Table 1. A mix of 7.84 kg ofcement, 11.76 kg of sand, 15.68 kg of rock and 2.74 kg of water, 570.24g of SP8 and 792 mL of microcapsules from example 1 were contained in aconcrete mixer and poured in a beam molders. After a day, samples werestripped from their molds, and submerged in water for 14 days to ensurefull curing.

Flexural Strength Test

Flexural strength procedure was adapted from ASTM C78/C78M-10 and BS1881: Part 118 with 22.5 cm×7.5 cm×3.5 cm dimension samples. Each samplewas subjected to an applied load of 0.50 mm/s in a 4-point bendinginstrument. Initial strength was determined by applying load untilsample reached failure. To induce micro-cracking, samples were loaded at80% of its initial strength, healed for several days and re-tested tofailure again.

Results of the initial and retested samples were an average of threespecimens as shown in FIG. 8. It shows that the concrete withmicrocapsules reaches at a flexural strength greater than the initialone after healing.

EXAMPLE 3

A self-healing concrete in cubes (15 pieces) with size of 10 cm×10 cm×10cm used for compressive strength testing were prepared using thecomponent ratio in Table 1. A mix of 8.91 kg of cement, 13.36 kg ofsand, 17.81 kg of rock and 3.12 kg of water followed by 648 g of SP8 and900 mL of microcapsules from example 1 were provided to a concretemixer. The mixture was poured on a cube molder and stripped after a day.Samples were submerged in water for 14 days to ensure full curing.

Compressive Strength Test

Compressive strength of self-healing concrete containing microcapsulewas compared with a normal concrete (control) before and aftermicrocracking occurs. The normal concrete as control was prepared as inexample 3 except the addition of SP8 and microcapsules.

Compressive strength procedure was adapted from ASTM C39/C39M-10 and BS1881: Part 116f for cube samples with 10 cm×10 cm×10 cm dimensions. Thetesting machine was equipped with s steel bearing platens with hardenedfaces larger than the size of the samples. The rate of load or stressranged from 0.2-0.4 MPa/s. Initial compressive stress was determinedwith three replicates by applying stress on samples until it reachedfailure. The measured values and the average value are presented in FIG.9. The average value is 68.1 MPa.

Succeeding experiments on example 3 were carried out by applying stressuntil sample reached a maximum load before failure. This was at 80% ofthe initial stress for 10 seconds to induce micro-cracking. After which,samples were submerged in water again for healing and retested tofailure again. Results of the initial and retested samples were anaverage of three specimens as shown in FIG. 10. The concrete withmicrocapsules recovered with a compressive strength exceeding itsinitial one.

Water Absorptivity

Water absorptivity procedure was adapted from ASTM C642-06 and BS 1881:Part 122 with 10 cm×10 cm×10 cm dimension samples from example 3 andcontrol. Initial water absorptivity of the self-healing concrete andcontrol were shown in FIG. 11. The average value of self-healingconcrete is 55% less than the control sample.

Samples were first cured in a drying oven for 72 hours and cooled at for24 hours at room temperature. Samples were weighed and immediatelyimmersed in a water tank. Samples were removed from the tank after 30minutes, surface dried and weighed again. Results of the waterabsorptivity were an average of three replicates as shown in FIG. 12.

Certain features of the application have been described with referenceto example embodiments. However, the description is not intended to beconstrued in a limiting sense. Various modifications of the exampleembodiments, as well as other embodiments of the application, which areapparent to persons skilled in the art to which the application pertainsare deemed to lie within the spirit and scope of the application.

What is claimed is:
 1. A process for preparing a composite materialcomprising: emulsifying a colloidal silica core in presence of a solventand a mixture of at least two surfactants at an emulsifying speed withina range of 400-600 rpm, wherein the mixture of at least two surfactantsis having a hydrophile-lipophile balance (HLB) within a range of3.0-5.0; and encapsulating the emulsified colloidal silica core with apolymer shell to form a plurality of microcapsules.
 2. The process ofclaim 1, wherein the weight percentage of silicon dioxide in thecolloidal silica ranges from about 40% to about 50%.
 3. The process ofclaim 1, wherein the surfactants are selected from the group consistingof poly(ethylene glycol)-400 dioleate, poly(ethylene glycol)-8 dioleate,sorbitan laurate, poly(ethylene glycol)-40 sorbitan peroleate, lecithinglycol distearate, sorbitan trioleate and propylene glycol isostearate.4. The process of claim 1, wherein the surfactants comprise one selectedfrom the group consisting of poly(ethylene glycol)-8 dioleate, sorbitanlaurate, poly(ethylene glycol)-40 sorbitan peroleate and lecithin, andanother selected from the group consisting of glycol distearate,sorbitan trioleate and propylene glycol isostearate.
 5. The process ofclaim 1, wherein the surfactants comprise poly(ethylene glycol)-400dioleate and sorbitan trioleate.
 6. The process of claim 1, wherein thepolymeric shell comprises at least one selected from the groupconsisting of polyurethane, polyurea, poly(urea-urethane), polystyrene,and urea-formaldehyde polymer.
 7. The process of claim 1, wherein themicrocapsule comprises a size of about 100 microns to about 200 microns,and the polymeric shell comprises a thickness of about 30 microns toabout 50 microns.
 8. The process of claim 1, wherein the encapsulatingstep comprises a stirring step at a speed of about 700-950 rpm.
 9. Theprocess of claim 1, further comprising adding surfactants.
 10. Theprocess of claim 1, further comprising embedding a plurality of themicrocapsules in a concrete mixture.